Colorectal cancer is a major public health problem as one in 20 Americans will acquire this disease, the cause of which is unknown. Therapeutic advances have not improved survival in recent decades. Improved survival depends on early diagnosis. As colon cancers bleed at early stages, measurement of fecal blood appears to be the most cost-effective screening approach. However, available tests, largely guaiac-based, are chemically unreliable. A new assay, HemoQuant, measures the fluorescence of heme-derived porphyrin, and appears to have major biochemical and clinical advantages over guaiac tests. Preliminary studies have shown that HemoQuant highly discriminates symptomatic colorectal cancer from healthy subjects. The aims are to: (1)\assess the validity of HemoQuant in detecting asymptomatic colorectal neoplasms and compare this with Hemoccult results; (2)\determine if measurement of that portion of heme degraded to porphyrin during intestinal transit predicts the anatomic level of bleeding lesions; (3)\describe the bleeding patterns of colorectal neoplasms; and (4)\optimize the HemoQuant screening process. The North Central Cancer Treatment Group (NCCTG) provides a large asymptomatic population at higher-than-average risk for colorectal cancer: an estimated 1200 patients with prior resection of a colon cancer and 4000 first order relatives of these patients. Subjects will be screened with Hemoccult and HemoQuant at 6-month intervals and either evaluated with routine proctoscopy and colon X-rays (post-operative patients) or evaluated based on abnormal screening results over 5 years. This study represents the final phase of HemoQuant testing required to justify its application in a controlled screening trial of a general population. (3)